Leak primarily for mass spectrometers



Oct. 18, 1955 W. H. BENNETT LEAK PRIMARILY FOR MASS SPECTROMETERS Filed Aug. 14, 1951 SUBJECT 70 VACUUM MASS 6P6 creams 72/? MA/N VAcuu/v 0F D/FFUJ/O/V PUMP DIFFUJ/O/V PUMP FORE VACUUM OF THE FORE VACUUM PUMP Pump IN VE NTOR BY @704 p/ ATTORNEY3 United States Patent Ofiice 2,721,270 Patented Oct. 18, 1955 LEAK PRIMARILY FOR MASS SPECTROMETERS Willard H. Bennett, Fayetteville, Ark.

Application August 14, 1951, Serial No. 241,813

Claims. (Cl. 25041.9)

This invention relates to apparatus for slowly admitting gas into an evacuated envelope and more particularly (although not exclusively) to a leak for a mass spectrometer.

In a mass spectrometer it is desirable to admit gas into the envelope at a very slow rate. Moreover, it is often desirable to admit it from a source or tank which is at atmospheric pressure whereas the envelope is at a very low pressure. It is desirable to admit the gas at a very slow controlled rate.

Where the gas to be admitted into the spectrometer comprises a mixture of gases of different weights, difliculty is encountered because the lighter gases of the mixture.tend to flow through a small hole more readily than do the heavier gases, and hence the gas fed into the spectrometer is not a true sample of the original mixture. There have been a number of efforts addressed at overcoming this difficulty, including admitting the original gas very slowly into a flask, and from that flask into another, etc., each flask having a lower pressure. This procedure is very slow and is undesirable because mass spectrometer readings are often used to control chemical processes and it is desirable to have an analysis as soon as possible in order that any deviations from normal can be promptly corrected. Moreover, if the readings show a deviation from normal it is desirable that the spectrometer respond promptly to any changes made in the gases fed thereto in order that the operator can immediately see the effect of any adjustments that he makes in the chemical process being analyzed.

The device for admitting a small quantity of gas into a spectrometer is known as a leak. Many problems other than those enumerated above are known to those skilled in the art and will not be recited in detail, it being suflicient to say that the present invention has for its primary object the provision of a simple leak which admits a predetermined limited flow of gas into the spectrometer, without delay, and without large errors due to lighter gases appearing in the spectrometer in a greater percentage than they were in the original mixture.

Other objects and further advantages of the invention will appear as this description proceeds.

An extremely slow leak normally makes use of a very small opening and such an opening is readily obstructed by condensed vapors, corrosion or fine dust particles, resulting in difliculty in regulating the leak rate. According to the present invention the mixture of gases at atmospheric pressure is first passed through a needle valve with a much larger opening than would ordinarily be used. The flow through this restriction is primarily viscous because the mean free path of the molecules in gases at pressures of the order of magnitude of atmospheric pressure is several orders of magnitude smaller than any aperture that can be made to stay clear of obstructions and condensations. This restriction is connected through a pipe to a long narrow restriction and leading from the side wall of this pipe ahead of the restriction is a large pipe to a vacuum pump so that excess gases passing through the needle valve will be removed, and especially the lighter gases will be removed, thus compensating for the fact that an excess percentage of light Weight gases may have passed the needle valve. The combination of the needle valve and exhaust pipe alone is, however, not effective, but is effective if connected to the spectrometer through the small elongated tube as aforesaid. The small tube is distinguished from the perforated diaphragms of the prior art because in the case of the small elongated tube the diameter of the opening may be much larger than in the case of a perforated diaphragm since resistance to the gas flow is also provided by the length of the tube.

Figure 1 of the drawing is a sectional view of the preferred form of the invention, and Figure 2 is a sectional view-of part of a modified form of the invention.

In the drawing an inlet pipe 1 connects to the source of gas to be analyzed which may be at, or more often above, atmospheric pressure. The inlet pipe has a vent 2 for discharging part of the gas in order to keep the gas adjacent the needle valve in motion at approximately atmospheric pressure and to pass the gas along rapidly in order that gases drawn from a reaction chamber in which the composition is changing will arrive over the opening of the needle valve soon after emerging from the reaction chamber. The vent 2 may be connected to a flue, or if the gas is combustible the vent 2 may constitute a burner. The inlet pipe 1 is connected to the valve seat 4 into which needle valve 5 may be moved. The needle valve 5 is connected to bellows 6 and may be moved up and down by bolt 7, which is threaded to the rigid fixed support 8, and may be rotated by the thumb knob 9. The gas passing the needle valve enters pipe 10 which is connected by pipe 11 to the fore-vacuum pump 12 of the diffusion pump 13 which in turn is connected by pipe 14 to the mass spectrometer 15, to constantly exhaust the latter. The pipe 10 has an elongated restriction 16 which is about three centimeters long and has an inside diameter far smaller than the inside diameters of pipes 10, 11 and 14 but large enough to give viscous flow. For example, a restriction of 0.3 millimeter diameter will give viscous flow and somewhat smaller diameters may be used in most cases and still have viscous flow.

By way of background, the mass spectrometer 15 may be briefly described as follows. It is a device enclosed in an envelopewhich by electrical means will analyze the gas fed thereinto producing information as to the type and quantity of gases in the mixture being analyzed. The mass spectrometer may be of the type described in my article appearing in the February 1950 issue of the Journal of Applied Physics, volume 21, No. 2, pages 143 to 149. Alternatively it may be of any other type. The mass spectrometer has an inlet pipe 17 for admitting at a very slow rate the gas to be analyzed and it also has an outlet pipe 14 which connects to a source of Very high vacuum.

The source of very high vacuum is illustrated as a conventional mercury diffusion pump having a main vacuum line 14 which operates at a very high vacuum (very low actual pressure in mass spectrometer 15). The rnrecury diffusion pump 13 also has an integral part thereof a fore-vacuum pump which does not have as high a vacuum as the main vacuum line 14.

Operation The gas to be analyzed flows into pipe 1 and through needle valve 4-5 into the pipe 10. The needle valve is open far more than is necessary to admit the proper amount of gas to the spectrometer 15. The major portion of the gas admitted through valve 4--5 is drawn through pipe 11 by the vacuum pump 12 and exhausted by the latter. Some of the gas leaks through the small tube 16 into pipe 17 and thence into the spectrometer 15 main suction pipe 14 of the mercury diffusion pump.

After a short period of operation the apparatus will reach a steady state flow condition during which some of the :lighter gases of the mixture will flow through the valve 4-5 more readily than the heavier ones. However, this error is not as great as it would have been if valves and/or perforated diaphragms (of the prior art) were used alone, due to the substantial flow allowed through the valve 4& The fore-vacuum pump acting through pipe '11 reduces the pressure in pipe It) and the vacuum in pipe 11 tends to draw oil lighter gases from pipe more readily than'it draws oil the heavier ones. The pressure in spectrometer 15 is less than that in pipe 19, hence the gases tend 'to move through the elongated small tube 16. However, the physical cfifects are diflercut for the gas flowing in the elongated tube 16 than they are with a perforated disc, under the circumstances of the peculiar combination herein claimed, and the gas flow leaving small tube 15 is at a slow rate and is fairly representative of the original sample so far as proportions of the constituent gases are concerned. With this combination the extremely small openings and cumbersome and slow gas handling systems of the prior art are avoided, since the present combination uses larger openings at valve 4-5 and at tube 16 than are customary in prior art devices for metering the sample gas into the spectrometer at a very slOw rate. In other words, the flow through valve 45 and tube 16 is essentially in my device.

If it is desired to analyze gases from reaction chambers or otl er containers at reduced pressure the vent 2 may be closed and the needle valve 4, 5 opened sufliciently to keep the pressure in pipe 10 at the proper value to enable gas to pass constriction 16 into pipe 17 at the proper pressure for analysis.

If it is desired to analyze a sample which is a liquid the sample may be held above the needle valve opening as shown in Figure 2 in which case the sample begins to pass the needle valve as a liquid but is vaporized part way through the needle valve opening. In this case, any tendency for the lighter components to vaporize preferentially is completely compensated by the increased concentration of the heavier components left behind at the place where vaporization is taking place.

I claim to have invented:

1. In combination, evacuated means and a system for feeding gases to said means comprising a pipe leading the gases to the evacuated means, said pipe having a restriction therein, means for evacuating the pipe upstream of the restriction to a degree of vacuum less than that of the evacuated means, and a second restriction comprising a valve in the pipe upstream of the region of the evacuation thereof, the cross-section of flow through said restrictions being very small as compared to the cross-section of flow elsewhere in the system but large enough to provide viscous flow therethrough whereby the pressure is greatly lowered on the down-stream side of said second restriction to thus effect a large pressure drop across the second restriction.

2. in combination, evacuated means and a system for feeding gases to said evacuated means comprising conduit means having first and second restrictions for leading the gases to said evacuated means both of which restrictions are long as compared to their transverse dimensions and both of which have such large cross section that the flow therethrough is primarily viscous, and means for exhausting the conduit means between the two restrictions to a degree of evacuation less than that of the evacuated means.

3. The combination of claim 2 including means for feeding a liquid into the first'restriction and wherein said first restriction is so small that the liquid is gasified as it enters the first restriction.

4. In combination, a mass spectrometer having an inlet and an "output and means for continuously supplying the spectrometer with gases to be analyzed comprising a source of said gases at a pressure in the order of magnitude of an atmosphere, conduit means connecting said source to said inlet and being provided with tWo restrictions which are long as compared to their transverse dimensions but both of which are large enough to provide viscous flow therethrough, and vacuum producing means connected to the conduit means between said restrictions as well as to the outlet of the mass spectrometer for evacuating the latter more than the former, said restrictions having such small openings that they in combination with said vacuum producing means reduce the pressure from that of the source to that of the mass spectrometer.

5. The combination of claim 4 in which said conduit means includes means for varying the cross-section of the first restriction.

Gas Flow in the Mass Spectrometer, by Honig, Journal of Applied Physics, vol. 16, November 1945, pages 646654.

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